add bsn_log extension

This provides a mechanism for either the controller or switch to request the
other side to output a log message, subject to the configured loglevel. This is
useful for log (and OpenFlow pcap) correlation.
bsn_log: add a comment about the string format
Merge into master from pull request #288:
add bsn_log extension (https://github.com/floodlight/loxigen/pull/288)
Add vrf counter request/reply.

Add OFP_BSN_VLAN_ALL constant to request counters for all vlans.
OFP_BSN_VRF_ALL should be 32 bits in length.
Change wire_type of VRF_ALL to uint32_t.
Merge into master from pull request #291:
Add vrf counter request/reply. (https://github.com/floodlight/loxigen/pull/291)
add bsn_action_checksum extension
add bsn_require_vlan_xlate extension
add bsn_vlan_xlate_port_group_id extension